Spectroscopy and Dynamics of Semiconductor, Metal, and Carbon Clusters using Photoelectron Imaging

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

This project addresses valence band spectroscopy and electronic relaxation dynamics in semiconductor, metal, and carbon clusters investigated by two methods: anion photoelectron imaging using vacuum ultraviolet (VUV) light sources, and femtosecond time-resolved photoelectron imaging. The VUV experiments will focus on mapping out the valence band structure of size-selected indium phosphode and silicon semiconductor clusters. Photodetachment at high energies, particularly at 10.5 electron-volts, will probe more deeply into the valence bands of these species than has been previously possible. Detection of the ejected photoelectrons using velocity-map imaging (VMI) will provide simultaneous determination of photoelectron kinetic energy and angular distributions, while also discriminating between direct photodetachment and thermionic emission. The time-resolved photoelectron imaging (TRPEI) experiments will track relaxation pathways in electronically excited clusters. TRPEI will be used to follow internal conversion and Auger decay dynamics in mercury clusters. It will also be applied to electronic relaxation dynamics in indium phosphide cluster anions, and in pure and metal-doped fullerides. Success on this project requires a combination of critical and independent thinking along with expertise in a wide range of laboratory instrumentation, including vacuum, lasers, electronics, and computer programming. Students trained in these broad areas are highly competitive in the job market. %%% The proposed research program in cluster science focuses on how the properties of matter, particularly semiconductors and metals, evolve between the molecular and bulk size regimes. As such, it provides a fundamental framework for understanding the foundations of nanoscience and nanotechnology. The increasing emphasis on nanotechnology in basic research and society as a whole has stimulated much interest in fundamental questions of cluster science. While nanoscience focuses on how the properties of bulk materials differ in the nanoscale regime, cluster science has traditionally focused on how the properties of atoms and molecules evolve upon aggregation. This approach, in which one investigates changes in the geometry and spectroscopy of clusters with increasing size, has been extremely productive. However, in recent years, there has been much interest in a parallel question, namely how large does a cluster have to be in order to observe the analog of phenomena normally associated with bulk materials, such as band structure in semiconductors, metal-insulator transitions, electron solvation in liquids, and thermionic emission. The proposed research will address many of these issues. It will also contribute significantly to the scientific infrastructure of the U.S. by providing state-of-the-art research training to young researchers at the undergraduate, graduate, and post-doctoral levels. This NSF project is being co-funded by the Chemistry Division and the Division of Materials Research.